Acidic air pollutants dissolve iron in aerosols and fertilize the ocean.

Abstract. Nitrate compounds have received much attention because of their ability to alter the hygroscopic properties and cloud condensation nuclei (CCN) activity of mineral dust particles in the atmosphere. However, very little is known about specific characteristics of ambient nitratecoated mineral particles on an individual particle scale. In this study, sample collection was conducted during brown haze and dust episodes between 24 May and 21 June 2007 in Beijing, northern China. Sizes, morphologies, and compositions of 332 mineral dust particles together with their coatings were analyzed using transmission electron microscopy (TEM) coupled with energy-dispersive X-ray (EDX) microanalyses. Structures of some mineral particles were verified using selected-area electron diffraction (SAED). TEM observation indicates that approximately 90% of the collected mineral particles are covered by visible coatings in haze samples whereas only 5% are coated in the dust sample. 92% of the analyzed mineral particles are covered with Ca-, Mg-, and Na-rich coatings, and 8% are associated with K-and S-rich coatings. The majority of coatings contain Ca, Mg, O, and N with minor amounts of S and Cl, suggesting that they are possibly nitrates mixed with small amounts of sulfates and chlorides. These nitrate coatings are strongly correlated with the presence of alkaline mineral components (e.g., calcite and dolomite). CaSO 4 particles with diameters from 10 to 500 nm were also detected in the coatings including Ca(NO 3 ) 2 and Mg(NO 3 ) 2 . Our results indicate that mineral particles in brown haze episodes were involved in atmospheric heteroCorrespondence to: L. Y. Shao (shaol@cumtb.edu.cn) geneous reactions with two or more acidic gases (e.g., SO 2 , NO 2 , HCl, and HNO 3 ). Mineral particles that acquire hygroscopic nitrate coatings tend to be more spherical and larger, enhancing their light scattering and CCN activity, both of which have cooling effects on the climate.

No abstract

Airborne aerosol collections were performed in urban areas of Beijing that were affected by regional brown haze episodes over northern China from 31 May to 12 June 2007. Morphologies, elemental compositions, and mixing states of 810 individual aerosol particles of different sizes were obtained by transmission electron microscopy coupled with energy‐dispersive X‐ray spectrometry. The phases of some particles were verified using selected‐area electron diffraction. Aerosol particle types less than 10 μm in diameter include mineral, complex secondary (Ca‐S, K‐, and S‐rich), organic, soot, fly ash, and metal (Fe‐rich and Zn‐bearing). Most soot, fly ash, and organic particles are less than 2 μm in diameter. Approximately 84% of the analyzed mineral particles have diameters between 2 and 10 μm, while 81% of the analyzed complex secondary and metal particles are much smaller, from 0.1 to 2 μm. Trajectory analysis with fire maps show that southerly air masses arriving at Beijing have been transported through many agricultural biomass burning sites and heavy industrial areas. Spherical fly ash and Fe‐rich particles were from industrial emissions, and abundant K‐rich and organic particles likely originated from field burning of crop residues. Abundant Zn‐bearing particles are associated with industrial activities and local waste incinerators. On the basis of the detailed analysis of 443 analyzed aerosol particles, about 70% of these particles are internally mixed with two or more aerosol components from different sources. Most mineral particles are covered with visible coatings that contain N, O, Ca (or Mg), minor S, and Cl. K‐ and S‐rich particles tend to be coagulated with fly ash, soot, metal, and fine‐grained mineral particles. Organic materials internally mixed with K‐ and S‐rich particles can be their inclusions and coatings.

Emissions from agricultural biomass burning (ABB) in northern China have a significant impact on the regional and global climate. The monthly average aerosol optical depth (AOD) at 550 nm in northern China in 2007 had a maximum of 0.7 in June. The AOD measurements are consistent with regional brown hazes that occurred at that time, which was a period of severe aerosol pollution. Aerosol particles were collected in urban Beijing from 12 to 30 June 2007, during a period of high haze, and studied using transmission electron microscopy with energy-dispersive X-ray spectrometry. The dominant particle types collected in the fine fraction (diameter <1 μm) were ammonium sulfate, soot, K₂SO₄, KNO₃, and organic matter, except that the K salts were minor between 21 and 30 June. K-rich particles as tracers of biomass burning, together with wildfire maps, show that intense regional ABB in northern China contributed significantly to the regional haze between 12 and 20 June. We therefore grouped the episodes into type-1 and -2 haze, with the former occurring between 12 and 20 June and the latter between 21 and 30 June. After long-range transport, ABB particles in the type-1 haze exhibited marked changes in morphology, composition, and mixing state. KCl particles were absent, presumably having been converted by heterogeneous reactions to K₂SO₄ and KNO₃. Soot particles were mixed with the other particle types. Abundant organic matter and soluble salts emitted by ABB increased their sizes during transport and resulted in more hygroscopic aerosol particles in downwind areas, becoming additional cloud condensation nuclei. The high AOD (average value 2.2) in Beijing during 12 to 20 June is partly explained by the hygroscopic growth of fine aerosol particles and by the strong absorption of internally mixed soot particles, both coming from regional ABB emissions. Therefore, it is important to consider the origins of the haze, which in turn leads to the different particle types

[1] Asia dust storm (ADS) samples were collected during two severe dust storm episodes in Beijing, China. High-resolution field emission scanning electron microscopy (FESEM) and environmental scanning electron microscopy-energy dispersive X-ray detector (ESEM-EDX) coupled with an image analysis system were used to study the morphology, chemical compositions, and number-size distributions of ADS particles. FESEM analysis indicated that Beijing ADS samples were mainly composed of irregularly shaped mineral particles. More than 50% of the ADS particles were larger than 1 mm, measured as equivalent spherical diameter (ESD). Image analysis showed that when the dust storm episodes ended in Beijing, the number of anthropogenic particles, such as soot aggregates and coal fly ash, in the samples increased, and the number of mineral particles decreased. Analysis of individual ADS particles showed that about one fifth of all the particles were mineral aggregates, and at least one fourth of the particles contained sulfur. X-ray diffraction (XRD) was used to quantify the phase and the clay mineral compositions of ADS samples. Phases in the ADS sample collected during the 20 March 2002 dust storm episode included clay minerals, noncrystalline materials, quartz, calcite, plagioclase, potassium feldspar, pyrite, hornblende, and gypsum in descending order. Clay minerals were mainly illite/smectite mixed layers (78%), followed by illite (9%), kaolinite (6%), and chlorite (7%). Particulate matter (PM) less than 10 mm were enriched with clay minerals and deficient with quartz by mass compared with the total suspended particulates collected during an ADS episode. The PM less than 10 mm collected during the two severe dust storm episodes was characterized by the absence of dolomite, high quartz/clay ratio, and dominance of illite/ smectite mixed layers in clay minerals.Citation: Shi, Z., L. Shao, T. P. Jones, and S. Lu (2005), Microscopy and mineralogy of airborne particles collected during severe dust storm episodes in

No abstract

No abstract